1. Field of the Invention
The present invention relates to an apparatus for detecting a rotational speed of an element such as a photosensitive drum, a carrying roller, etc. which may be included in devices such as a copier, a printer, etc.
2. Discussion of the Background
FIG. 8 is a view showing an outline structure of a background color image forming apparatus. In this apparatus image forming portions 1Y, 1M, 1C, 1Bk which form different color images (e.g., respectively of yellow, magenta, cyan, black) are placed in a row along a carrying belt 7 for carrying recording paper sheets. The carrying belt 7 is spanned between carrying rollers 8 and 9, each carrying roller 8, 9 having a driving roller at one side and a free roller at another side. Accordingly, the carrying belt 7 is rotated in the direction indicated by the arrow direction by the carrying rollers 8, 9. A paper tray 12 for storing stored recording paper sheets 11' is placed under the carrying belt 7.
An upper recording paper sheet 11 stored in the paper tray 12 is drawn to the carrying belt 7 by a carrying mechanism (not shown), and is absorbed to the carrying belt 7 by an electrostatic force, and is drawn when an image forming operation is to be executed. The drawn recording paper sheet 11 is carried to first image forming portion 1Y (e.g., yellow) and a yellow image is formed on the drawn recording sheet at this position. The first image forming portion 1Y includes a photosensitive drum 2Y, a charger 3Y placed around the photosensitive drum 2Y, an exposing unit 4Y, a developing unit 5Y, and a cleaning unit 6Y. After the photosensitive drum 2Y is uniformly charged by the charger 3Y, a surface of the photosensitive drum 2Y is exposed to a laser beam in response to a color image by the exposing unit 4Y, the developing unit 5Y, and the cleaning unit 6Y. That is, after the photosensitive drum 2Y is uniformly charged by the charger 3Y, a surface of the photosensitive drum 2Y is exposed to a laser beam in response to a color image by the exposing unit 4Y, and thereby an electrostatic image is formed on the photosensitive drum 2Y. The formed electrostatic image is then developed by the developing unit 5Y, and a toner image is thus visualized on the photosensitive drum 2Y. The toner image is then transferred to the recording paper sheet 11 at a contacting position of the photosensitive drum 2Y and the recording paper sheet 11 on the carrying belt 7 (transferring position) by a transferring unit 13Y, and thereby a single color image (e.g., yellow image) is formed on the recording paper 11.
Residual toner remaining on the surface of the photosensitive drum 2Y is cleaned by the cleaning unit 6Y, and the photosensitive drum 2Y is thus prepared for a next image forming operation. In this situation, the recording paper sheet 11 having the transferred single color image (e.g., yellow) from the first image forming portion 1Y is carried to a second image forming portion 1M (e.g., magenta). In this situation, a next toner image (e.g., magenta) formed on the photosensitive drum 2M is transferred to the recording paper 11 sheet to be superposed on the first (e.g., yellow) toner image by the same operations as discussed above. Furthermore, the recording paper sheet 11 is then carried to the third image forming portion 1C (e.g., cyan), and then the fourth image forming portion 1B (e.g., black), and the same operations are then repeated. With these operations, four toner images (e.g., yellow, magenta, cyan, black) are transferred to the paper sheet 11 and thus a full color image is formed on the paper sheet 11. The paper sheet 11 after passing by the fourth image forming portion 1Bk is then removed from the carrying belt 7 and the image thereon is fixed by a fixing unit 14, and the paper sheet 11 is then discharged to a discharging tray 15.
The above image forming apparatus which places four image forming portions 1Y, 1M, 1C, 1Bk in one row has advantages that a printing speed is faster than other types of apparatus. However, this type of image forming apparatus has a defect in that slipping among the four color images easily results after forming the four color images on recording paper sheets over and over again, and thereby an image quality deteriorates. To solve this problem, a resist forming correction can be carried out for avoiding mispositioning of images by forming a resist mark 17 on the carrying belt 7, as shown in FIG. 9. A positioning of the resist mark 17 for each image formed is detected by a resist mark sensor 10, and positioning is carried out by adjusting a timing of the laser beam output from each of the exposing units 4 based on the detections of the resist marks 17.
However, the background resist positioning correction is limited to positional correction for static resist mispositioning as a variation of attaching positional accuracy in each unit. In reality, however, mispositioning appearing on the output color image is not limited to the above static resist mispositioning. For example, if there are variations in rotating speeds in each photosensitive drum 2, or variations in carrying speeds of the carrying belt 7, or variations in a driving gear pitch, etc., an irregular pitch of scanning lines in a scanning direction with a special frequency may be produced. This irregular pitch can appear on the output image as an irregular density called "BANDING". Namely, a dynamic mispositioning can appear on an output image.
As a correcting method for this mispositioning with a special frequency, as shown in Japanese unexamined patent (KOKAI) No. 7-225544, a method which detects a speed variation of a photosensitive drum and which controls a record timing in response to the detected speed can be implemented. Further, as another method, a method of correcting a rotational driving of a photosensitive drum in response to a detected speed variation of the photosensitive drum can be implemented. The above methods will now be explained with reference to FIG. 9 which shows a partial structure of a background image forming apparatus. Rotary encoders 19Y, 19M, 19C, 19BK for detecting a rotating speed of the photosensitive drums 2Y, 2M, 2C, 2BK are attached to a rotating shaft of each respective photosensitive drum 2Y-2BK. A detected speed variation by each rotary encoder 19 is fed back to a rotating control circuit of a respective photosensitive drum driving motor 18Y, 18M, 18C, 18BK which is attached to a rotating shaft of each respective photosensitive drum 2 at an opposite side to the respective rotary encoder 19.
As mentioned above, when detecting a rotational speed of the photosensitive drums 2, rotary encoders 19 are usually utilized, which rotary encoders 19 are expensive elements. If a heightened accuracy of detecting the rotational speed of each photosensitive drum 2 is required, a number of slits in a disc of each rotary encoder 19 can be increased, but however this results in a cost of the rotary encoders 19 increasing. In the image forming operations of the toner type image forming apparatus as shown in FIG. 8 and FIG. 9, because of having to attach a rotary encoder 19 to each of the four photosensitive drums 2, a cost of the rotary encoders 19 can not be ignored.
Furthermore, even if using an accurate rotary encoder 19, the attaching accuracy of the rotary encoder 19 can be very important. Namely, when a rotating axis of the photosensitive drums 2 and a rotating axis of the respective rotary encoders 19 do not correspond accurately, it is impossible to detect a rotational speed accurately. For example, Japanese unexamined patent publication (KOKAI) No. 7-306612 notes that if an axis of the rotary encoder 19 does not properly correspond to an axis of the photosensitive drum 2 perfectly, it is impossible to detect a rotational speed of the photosensitive drum 2 accurately. When there is a difference in a position between a rotating axis of the rotating encoder 19 and an axis of a rotating body as a detecting object, an accuracy of a detecting speed can worsen.
Now, a case of a difference in a position between a rotating axis of a rotating body, such as a photosensitive drum, and a rotating axis of a rotary encoder for detecting speed will be considered. FIG. 10 is a view showing a general rotary encoder which includes slits 21 with a narrow pitch formed on a rotating disc 20. The rotating disc 20 rotates around a rotating axis 24. A radiant element 22 radiates rays to pass through the slit 21 of the rotating disc 20, which rays are detected by a photodetector 23. There are many encoders which have a structure of placing fixed slits 21 between a rotating disc 20 and a photodetector 23. In this type of encoder, the radiant element 22 and the photodetector 23 detect variations in speed by detecting the passing of the slits 21 in a placed position.
FIG. 11 is a view showing a situation of how an error arises between a rotating axis O1 (rotating center) of a rotating body and a rotating body and a rotating axis O2 (circle center) of a rotating disc 20 of the encoder. If a mispositioning occurs between the rotating axis 24 of the rotating disc 20 and a rotating axis of a rotating body, the rotating disc 20 does not accurately rotate around the rotating axis O1 of the rotating body, namely it rotates eccentrically. A distance from the rotating axis O1 of the rotating body to the detecting position P is D, a difference in position between the rotating axis O1 and the axis O2 of the rotating disc 20 is A, and the rotating body rotates with a constant angular velocity W0. In this situation, if D=10 (mm), A=0.1 (mm). W0=2.pi.(rad/sec), a number of slits 21 passing the detecting position P is as shown in FIG. 12(a). In this situation, as the rotating disc 20, a rotary encoder which has 5000 slits per one round with equal pitch may be used. In FIG. 12(a), the dashed line shows a number of slits 21 passing the detecting position P when there is no mispositioning of the axes, and the solid line shows a number of slits passing the detecting position P when there is a difference in the axes positioning as shown in FIG. 11. FIG. 12(b) shows a difference (error number) between a passing slit number when there is a difference in the axes position and a passing slit number when there is no difference in the axes position. In this situation, about an eight slit difference (error number) is shown. When detecting a rotational speed of a rotating body of a detecting object, an error of passing slits at the detecting position appears as a detected speed error. FIG. 12(c) shows this resulting detected speed error in the slits 21 passing at the detecting position P (a speed error of .+-.1% is produced for an ideal speed (the dashed line)).
In the above-mentioned operation, an error of a detected speed by a positional difference between an axis of a rotating body and an axis of an encoder is explained. When detecting a rotational speed of a rotating object, such as a photosensitive drum, by an encoder, a true speed variation of the rotating body to be detected should be within 1%. As noted above, the mispositioning as shown in FIG. 11 results in a greater error. Therefore, a detected speed variation resulting from a positional difference (mispositioning) of the axes can not be ignored. Accordingly, for avoiding such a detecting error, high mechanical accuracy between a rotating axis of a photosensitive drum and an axis of an encoder is necessary. FIG. 12 shows a case in which a pitch of slits 21 is at an ideal equal interval. In the background art, a number of the slits 21 is a factor for calculating an angular velocity of the rotating body. Therefore, if there is a variation of the slit pitch, this may also result in an error in the detected speed.